Downhole fluid analysis is an important and efficient investigative technique typically used to ascertain characteristics and nature of geological formations having hydrocarbon deposits. In this, typical oilfield exploration and development includes downhole fluid analysis for determining petrophysical, mineralogical, and fluid properties of hydrocarbon reservoirs. Fluid characterization is integral to an accurate evaluation of the economic viability of a hydrocarbon reservoir formation.
Typically, a complex mixture of fluids, such as oil, gas, and water, is found downhole in reservoir formations. Wireline formation testing tools for formation fluid analysis are disclosed in, for example, U.S. Pat. Nos. 3,780,575, 3,859,851 and 6,476,384, the entire contents of which are hereby incorporated herein by reference.
Formation fluids under downhole conditions of composition, pressure and temperature typically are different from the fluids at surface conditions. For example, downhole temperatures in a well could range from 300 degrees F. When samples of downhole fluids are transported to the surface, change in temperature of the fluids tends to occur, with attendant changes in volume and pressure. The changes in the fluids as a result of transportation to the surface cause phase separation between gaseous and liquid phases in the samples, and changes in compositional characteristics of the formation fluids, among other variations in fluid properties.
As a consequence of shortcomings in surface analysis of formation fluids, recent developments in downhole fluid analysis include techniques for characterizing formation fluids downhole in a wellbore or borehole. In this, sampling tools for extracting samples of formation fluids from a borehole for surface analysis, such as the Reservoir Formation Tester (RFT) and Modular Formation Dynamics Tester (MDT) of Schlumberger, may include one or more fluid analysis modules, such as the Composition Fluid Analyzer (CFA) and Live Fluid Analyzer (LFA) of Schlumberger, for example, to analyze downhole fluids sampled by the tool while the fluids are still downhole.
In downhole fluid analysis modules of the type described above, formation fluids that are to be analyzed downhole flow past a sensor module associated with the fluid analysis module, such as a spectrometer module, which analyzes the flowing fluids by infrared absorption spectroscopy, for example. In this, an optical fluid analyzer (OFA), which may be located in the fluid analysis module, may identify fluids in the flow stream and quantify the oil and water content. U.S. Pat. No. 4,994,671 (incorporated herein by reference in its entirety) describes a borehole apparatus having a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra), in order to characterize the formation fluids.
In addition, U.S. Pat. Nos. 5,167,149 and 5,201,220 (both incorporated herein by reference in their entirety) describe apparatus for estimating the quantity of gas present in a fluid stream. A prism is attached to a window in the fluid stream and light is directed through the prism to the window. Light reflected from the window/fluid flow interface at certain specific angles is detected and analyzed to indicate the presence of gas in the fluid flow.
As set forth in U.S. Pat. No. 5,266,800 (incorporated herein by reference in its entirety), monitoring optical absorption spectrum of fluid samples obtained over time may allow one to determine when formation fluids, rather than mud filtrates, are flowing into the fluid analysis module. Further, as described in U.S. Pat. No. 5,331,156 (incorporated herein by reference in its entirety) by making optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
As discussed above, optical systems have been used by the oil industry in downhole conditions. A spectrometer of the type generally used in downhole tools is based on filter array (FA) architecture using optical bandpass filters. Spectrometer input light is distributed on an array of optical bandpass filters and the optical absorption of formation fluids is measured at a fixed number of discrete wavelengths which are limited to the number of filters. Filter spectrometers, however, are not suitable for precise measurements of the hydrocarbon spectra with high wavelength resolution. Since conventional spectrometers use optical bandpass filters to separate light into spectral components, the spectral resolution is not good. Therefore, precise spectrum analysis with high wavelength resolution is not possible with a conventional type of spectrometer.
Size and cost factors also play a role in the unsuitability of filter spectrometers for downhole analysis of hydrocarbon fluids. Conventional spectrometers tend to be large in size because a set of filters and lenses, and a photo detector are required for each measurement channel. Consequently, conventional optical bandpass spectrometers are expensive.
Furthermore, despite a conventional spectrometer's cost, the number of measurement channels is limited by the space available in a bandpass spectrometer designed for downhole use. Since a typical downhole tool has limited space, the size of a bandpass spectrometer that is required to measure a suitable range of wavelength spectra is a disadvantage for downhole use.
Optical spectrometers that use gratings are known for surface uses, such as in a laboratory setting, but to applicants' knowledge presently there is no suitable grating spectrometer for downhole use. In this, since typical downhole conditions, such as temperature, pressure, among others, are extremely harsh operating conditions for spectrometry, conventional surface-use grating spectrometers are not adapted for downhole fluid analysis in an oil field setting.
Although grating spectrometers have been proposed for downhole use, practical implementation of the proposed spectrometers has been difficult. Significant limitations exist in conventional grating performance in high temperature (HT) environments that are typically found downhole.